With the recent trend to higher-capacity information processing, various information recording technologies have been developed. Particularly, a surface recording density of an HDD (Hard Disk Drive) using the magnetic recording technology has continuously increased by a rate of approximately 100% a year. In recent years, an information recording capacity exceeding 250 GB per disk is required for a magnetic disk having a radius of 2.5 inches used in HDD or the like, and in order to meet such demand, realization of an information recording density exceeding 400 Gbits per 1 square inch is in demand. In order to achieve the high recording density in a magnetic disk used in an HDD or the like, magnetic crystal grains constituting a magnetic recording layer handling recording of an information signal need to be refined, and its layer thickness needs to be reduced at the same time. However, in the case of a magnetic disk of an in-plane magnetic recording method (also referred to as longitudinal magnetic recording method or horizontal magnetic recording method) having been merchandized, as the result of development of the refining of the magnetic crystal grains, a thermal fluctuation phenomenon in which thermal stability of the recording signal is damaged by a superparamagnetic phenomenon and the recording signal is lost begins to occur, which makes an obstructive factor to higher recording density of a magnetic disk.
In order to solve this obstructive factor, a magnetic recording medium of a perpendicular magnetic recording method has been proposed recently. In the case of the perpendicular magnetic recording method, different from the in-plane magnetic recording method, a magnetization easy axis of a magnetic recording layer is adjusted to be oriented in the perpendicular direction with respect to a substrate surface. As compared with the in-plane recording method, the perpendicular magnetic recording method can suppress the thermal fluctuation phenomenon, which is suitable for higher recording density. This type of perpendicular magnetic recording mediums include a so-called two-layer type perpendicular magnetic recording disk provided with a soft magnetic underlayer made of a soft magnetic body on a substrate and a perpendicular magnetic recording layer made of a hard magnetic body as described in Japanese Unexamined Patent Application Publication No. 2002-74648 (Patent Document 1).